Dog and Cat Creatinine Levels 101

If you visit the veterinarian regularly with your dog or cat, there is a good chance that you will hear the term “creatinine levels.” So what is creatinine, and why do we test for it?

Creatinine is a breakdown product of creatine phosphate, which is a normal component of muscle and is removed from the blood primarily by filtration through the kidneys. Veterinarians measure creatinine levels because it helps to identify kidney problems.

Creatinine levels have been used to evaluate kidney disease for a long time. There are limitations to the use of this test in the early diagnosis of kidney disease and in dogs and cats with loss of muscle mass. A new test, symmetric dimethylarginine (SDMA), has recently been developed for dogs and cats to support earlier and more reliable diagnosis of kidney disease. Your veterinarian will generally run these tests simultaneously to get an accurate assessment of how well your pet’s kidneys are working.

What can creatinine say about the kidneys?The primary job of the kidneys is to remove waste chemicals from the body and excrete them into urine. Veterinarians measure chemicals that should be appropriately removed by your kidneys to determine if the kidneys are doing their job properly. Therefore, kidney disease is generally detected and evaluated by measuring the levels of certain chemicals, like creatinine, in the blood or urine.

What could low creatinine levels mean?Low levels of creatinine can indicate poor muscle health or severe liver disease. Creatinine can also be lowered by increased metabolic states caused by hyperthyroid disease or extreme weight loss associated with muscle depletion. Lower than normal creatinine levels may occur when pets are fed extremely protein deficient diets resulting in poor muscle mass.

What could high creatinine levels mean?Increased levels of creatinine occur when the filtration rate of the kidneys is inadequate to remove this protein waste product, typically when more than 75% of normal function is lost, signifying advanced kidney disease. SDMA usually increases earlier.

Creatinine is not completely specific for kidney disease, either, since it can be affected by factors other than kidney function. Some heavily muscled dogs with normal kidney function have creatinine values that are higher than the laboratory's normal reference values. And a dog's creatinine may increase shortly after eating a meaty meal. These are some reasons why it is important to have your pet's SDMA levels tested along with creatinine, since SDMA is less influenced by non-kidney factors.

How is creatinine measured and interpreted?Creatinine is measured by chemical analysis at a laboratory. Since normal levels of creatinine are typically quite low and increases can occur incrementally with time, one individual blood level might not mean much to your veterinarian. Interpretation of changing creatinine levels over time often gives a more complete assessment of kidney function.

Glomerular filtration rate tests (GFR) are the reference method for testing kidney function. The amount of blood the kidneys can filter in 24 hours is estimated by measuring how efficiently a substance like creatinine is cleared from the body -- called creatinine clearance. Since GFR tests require collecting blood and/or urine several times over a 24-hour period they are inconvenient and can be stressful to patients. Single measurements of creatinine and SDMA are used instead to estimate kidney function -- the GFR.

Creatinine is a valuable test but is insufficient on its own to diagnose kidney disease when pets may be helped the most. By the time creatinine levels are increased in the blood, your cat or dog has already lost the majority of its kidney function, and it may not be reversible.